1. Field of the Invention
The present invention is directed to a circuit arrangement for generating a d.c. output voltage regulated to a constant value.
2. Description of the Prior Art
U.S. Pat. No. 3,890,559 discloses a circuit arrangement for generating a constant output voltage wherein two identical frequency converters feed a common load via decoupling diodes. Each of the two frequency converters is capable of supplying the load by itself. The frequency converters each comprise a regulator for regulating the output voltage to a constant value. The voltage regulator operates so that the deviation of an actual value voltage, generated by an arrangement for forming an actual value from a reference voltage, is minimized as far as possible. The regulator is composed of a differential amplifier having two emitter-coupled transistors and having a further transistor connected thereto. In the differential amplifier, the base of one transistor is connected to the tap of an ohmic voltage divider that lies at the load resistor to be supplied. The base of the other of the two transistors of the differential amplifier is connected to the tap of a voltage divider that contains a Zener diode as the reference voltage source.
The reference potential terminal of the regulator in this known circuit is directly connected to the load resistor. If a voltage arose at the load resistor comparable in size and applied in some other way to the user such as derived from another, identical power supply this would substantially completely block the frequency converter so that it would not be capable of feeding the load resistor in an adequately short time as warranted. An auxiliary circuit composed of a transistor, of a voltage divider and a further Zener diode therefore can assume the control, and then insures that a prescribed value of the output current is not downwardly transgressed. The transistor lying at the output of the differential amplifier, an optocoupler and the auxiliary circuit are matched such to one another in view of their temperature coefficients such that temperature-conditioned changes act opposite one another.
The arrangement which forms the actual value can also be fashioned such that the actual value for the regulator is formed as a mixed value of the voltage preceding and following the decoupling diode. When a low tolerance of the output voltage is required, then the part of the voltage following the decoupling diode must predominate. In this case, however, the power supply that becomes active first blocks the other. When the mixing ratio of the measured values preceding and following the decoupling diode is only selected as 1:1 in order to avoid this problem, then only a limited precision of the user voltage can be achieved, since the voltage drop at the decoupling diode is greatly dependent on the temperature and on the load current.